In addition to an optical recording medium such as a reproduction-only Digital Versatile Disc (DVD), a recordable DVD (e.g., DVD+RW, DVD+R, DVD-R, DVD-RW and DVD-RAM) has been put to practical use.
The DVD+R and DVD+RW lie at the extension of the conventional technologies for recordable compact discs (CD-R and CD-RWs), and to maintain the reproduction compatibility of the DVD+R and DVD+RW with a reproduction-only DVD, the DVD+R and DVD+RW are designed so that the recording density (e.g., track pitch and signal mark length) and the thickness of substrate are compatible with those of CDs. For example, a DVD+R is manufactured in the same manner as a CD-R that an information recording substrate, where a recording layer is formed on a substrate and a reflective layer on the recording layer, is laminated with another substrate of the same configuration as the same form as the first substrate. The recording layer usually comprises a dye material.
Regarding the CD-R, its recording layer also comprises a dye material. One of the features of the CD-R is to have a high reflectivity (65%) satisfying CD standards. To obtain a high reflectivity with the above-noted configuration, however, it is necessary that the recording layer satisfies a specific complex refractive index at a recording and reproducing light wavelength and that the light absorption characteristics of the dye is appropriate. These are the properties that are to be satisfied as well for DVDs.
To increase the recording capacity of the existing reproduction-only DVD, a reproduction-only DVD having two recording layers has been proposed. For example, FIG. 1 is a cross-sectional view schematically showing a layer composition of a DVD having two recording layers. A first substrate 101 and a second substrate 102 are laminated to each other through an intermediate layer 105 comprising a UV-cure resin. On the inner surface of the first substrate 101, a first recoding layer 103 is formed, and on the inner surface of the second substrate 102, a second recoding layer 104 is formed. The first recoding layer 103 is formed as a translucent film of a dielectric film. The second recoding layer 104 is formed as a reflective film of a metal film. On the first recoding layer 103, convexo-concave recording marks are formed, and by the effect of reflecting and interfering a reproduction laser, recording signals are read. In the DVD shown in FIG. 1, signals are read in the two recording layers; therefore, the DVD may obtain a memory capacity of up to 8.5 GB.
Each of the first substrate 101 and the second substrate 102 has a thickness of approximately 0.6 mm, and the intermediate layer 105 has a thickness of approximately 50 μm. The translucent film as the first recording layer 103 is formed so that its reflectivity is around 30%; a laser applied to reproduce the second recording layer 104 is attenuated in the first recording layer 103 by about 30% of the whole laser intensity due to reflection. Thereafter, the attenuated laser is reflected at the reflective film of the second recording layer 104, and after the laser is attenuated once again at the first recording layer 103, the laser emerges from the optical recording medium. By focusing the laser as a reproducing light on the first recording layer or the second recording layer and detecting the reflected light, signals of each of the recording layers may be reproduced. Meanwhile, regarding a DVD, a laser used for recording and reproducing has a wavelength of about 650 nm.
However, up to the present date, there only exist recordable DVDS, DVD+Rs, DVD-Rs and DVD+RWs with one recording layer readable from one surface. In order to increase the memory capacity of these optical recording media, it is necessary that the information is reproduced from the both surfaces for the following reason. A single-sided double-layer optical recording medium for recording and reproducing comprises a first recording layer and a second recording layer, and when a signal is being recorded by applying a laser that is brought to focus on the second recording layer, which is located more distant from the optical pickup, the light absorption and light reflection required for recording on the second layer are incompatible due to the attenuation of the laser in the first recording layer.
For example, Patent Literature 1 proposes a configuration wherein, in recording, it is possible from one side of the optical recording medium to write in two recording layers that comprise an organic dye, and in reproducing, it is also possible from one side of the optical recording medium to read out from the two recording layers. However, this literature only proposes a configuration where two existing types of substrates, one configured such that a laser is applied on the surface of the substrate for recording and the other configured such that a laser is applied from the surface of the recording layer, are laminated, and the above-noted problem regarding the absorption and reflection of light in the second recording layer remains unsolved. Also, this patent literature does not disclose any verification results of recording signals. In addition, there is a problem that it is difficult to form a recording mark on the second recording layer because the configuration of the recording medium is different from those of the conventional CD-R and DVD+R.
A conventional single-layer optical recording medium comprises a substrate, a pigment layer, a reflective layer and a protective layer in this order from the side of the incoming light. On the contrary, the second recording layer of a double-layer optical recording medium comprises a laminating layer, (an inorganic protective layer), a recording layer, a reflective layer and a substrate in this order from the side of the incoming light, wherein the light has been transmitted through the first recording layer, and the environment of recording mark formation in the recording layer (an adjacent layer) is different. Hence, there are difficulties in achieving recording and reproducing properties such as modulation and jitter which are compliant to DVD systems.
Further, Patent Literatures 2 and 3 propose an optical recording medium, wherein the second recording layer of a double-layer optical recording medium comprises a laminating layer, an inorganic protective layer, a recording layer, a reflective layer and a substrate in this order from the side of an incoming light transmitted through the first recording layer. However, these patent documents disclose only the existence and absence of a recording mark (modulation), and the proposals thereof are equivalent to the one described in Patent Literature 1, that is, a configuration where two types of substrates, one configured such that a laser is applied to the surface of the substrate for recording and the other configured such that a laser is applied to the surface of the recording layer, are laminated; and the problem regarding the recording and reproducing remains unsolved.
On the other hand, regarding the existing single-layer optical recording medium, Patent Literatures 4 to 7, for example, discuss a mass decrease property during the pyrolysis of a pigment used for a recording layer. However, in these prior art, the performance in the formation of recording marks in the second recording layer in the case of a single-sided double-layer optical recording medium is not yet sufficient. Hence, as it stands, more improvements are desired.
Patent Literature 1Japanese Patent Application Laid-Open(JP-A) No. 11-66622Patent Literature 2JP-A No. 2000-311384Patent Literature 3JP-A No. 2003-331473Patent Literature 4Japanese Patent (JP-B) No. 3074715Patent Literature 5JP-A No. 09-58123Patent Literature 6JP-A No. 10-188341Patent Literature 7JP-A No. 11-134708